1. Field of the Invention
The present invention relates to a damping force control type hydraulic shock absorber for use in a suspension system of a vehicle, for example, an automobile.
2. Description of the Prior Art
Hydraulic shock absorbers for use in a suspension system of an automobile or other vehicle include damping force control type hydraulic shock absorbers which are designed so that the level of damping force can be controlled properly in accordance with the road surface conditions, vehicle running conditions, etc., thereby improving the ride quality and steering stability of the vehicle.
Japanese Utility Model Application Public Disclosure (KOKAI) No. 58-70533 (1983) discloses one example of the conventional hydraulic shock absorbers of the type described above. The disclosed hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein, and a piston having a piston rod connected thereto. The piston is slidably fitted in the cylinder to divide the inside of the cylinder into two chambers. The two chambers are communicated with each other through first and second hydraulic fluid passages. The first hydraulic fluid passage is provided with a first damping force generating mechanism (comprising an orifice, a disk valve, etc.) for generating a relatively large damping force. The second hydraulic fluid passage is provided with a second damping force generating mechanism for generating a relatively small damping force and is also provided with a damping force control valve for opening and closing the second hydraulic fluid passage.
With the above arrangement, when the damping force control valve is open, the hydraulic fluid in the cylinder mainly flows through the second hydraulic fluid passage in response to the sliding motion of the piston caused by the extension and retraction of the piston rod, generating a relatively small damping force during both the extension and retraction strokes, thus providing "soft" damping force characteristics. When the damping force control valve is closed, the hydraulic fluid in the cylinder flows only through the first hydraulic fluid passage in response to the sliding motion of the piston caused by the extension and retraction of the piston rod, generating the relatively large damping force during both the extension and retraction strokes, thus providing "hard" damping force characteristics. In this way, the damping force characteristics can be changed over from one to the other by opening or closing the damping force control valve.
Japanese Patent Application Public Disclosure (KOKAI) No. 61-75008 (1986) discloses a hydraulic shock absorber in which the second hydraulic fluid passage for providing communication between the two chambers in the cylinder comprises two hydraulic fluid passages respectively having check valves, one of which allows flow of the hydraulic fluid in only one direction, while the other allows flow of the hydraulic fluid in only another direction reversed to the above. The prior art further includes a shutter having a pair of openings for controlling the passage area of each of the two hydraulic fluid passages, thereby enabling different damping force characteristics to be obtained for the extension and retraction strokes by controlling the passage areas of the two hydraulic fluid passages through the shutter.
However, these conventional damping force control type hydraulic shock absorbers suffer from the following problems.
In the hydraulic shock absorber disclosed in Japanese Utility Model Application Public Disclosure (KOKAI) No. 58-70533 (1983), the relationship between the damping force characteristics for the extension and retraction strokes remains unchanged even when the damping force characteristics are changed from "soft" to "hard" or vice versa. In the hydraulic shock absorber disclosed in Japanese Patent Application Public Disclosure (KOKAI) No. 61-75008 (1986), the passage areas of the two hydraulic fluid passages are controlled by using a pair of openings provided in the shutter. Therefore, the range of selectable combinations of damping force characteristics for the extension and retraction strokes is small.
However, it is necessary in order to improve the ride quality and steering stability of a vehicle to change over damping force characteristics in accordance with various road conditions, vehicle running conditions, etc. Therefore, it is impossible to meet the requirements satisfactorily with the damping force controllable range of the above-described conventional damping force control type hydraulic shock absorbers.